# Reduction drives

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#### orion

##### Well-Known Member
As many of your have concluded, auto conversions are a realistic substitute for our airplanes. Given the continuous and generally trouble free service in automotive applications, there is no reason to assume that an airplane application woud result in failure. As long as the user understands that the airplane application will operate the engine at a higher power level than the engine sees in a car, then all other factors should be pretty much equal.

The problem therefore is not so much the engine but more so the reduction drive we place between the engine and the prop. In our research we have of course found many potential suppliers of redrives however, when we analyzed them for their published performance, we determined most to be lacking. To compound the problem, we have also discovered that many of these drives were generally "eyeballed" with no real engineering behind them. We also found that those who developed these critical components had no real knowlege of drive design requirements, nor of the potential loads that a flight environment will impose on the drive's components.

For those interested, I've posted a couple of engine/reduction articles at our web site. If you go to www.oriontechnologies.net and go the the Papers and Articles page, there are a couple of articles that talk about several of the issues to consider in this arena.

One interesting aspect of this is the real attitude of many of us who are considering an engine option for our airplane. We udertook a survey of the industry back in the mid eighties, and a supplementary one on the mid nineties. What we found is that there is tremendous interest in alternative engines. Virtually all responses (over ninety percent) respoded with a strong interest in automotive applications, reduction drives and all associated technologies.

However, when asked what engine all these people were most likely to put in their own airplane project, virtually all said Lycoming and Continental.

Thus far, we have therefore concluded that if an engine/redrive package is to be successful in the market, it first must have a professional level of technical backing behind it, it must appear professionally developed, machined and built, and it must be flight-tested to the level that a certified engine would be. This does not mean that folks want these engines to be certified, it just means that a substantial amount of testing needs to be behind it before the customer base takes it seriously.

Also, the customer must get past the company glitz, in order to do a real evaluation as to the suitability of the package. There are several companies out there providing redrives or drive/engine packages that look slick and professional - they are very well machined, anodized in pretty colors, and supported with slick sales brochures. But unfortunately they have virtually no real engineering, nor history, behind them.

There are a few good ones out there but discerning between the good and the bad is difficult. In essence, what this does is force the customer into the position of being a test pilot, something I'm sure most of us don't want to be. And so, most still consider the overpriced tractor-level technology standard to be the only choice for our airplane.

I think most of us would like this to change, but it will require a responsible approach to the development, and of course, a few dollars to make it work.

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#### mel

##### Member
redrives

Hi This is Mel in Orange Park Fl. You are prob right. I have no engineer training. Still I putting on my engine a redrive using quick change gears from cars. They are strong and tuff. The only grief is that I dont know how to hook up a Constant speed prop to it but will figure it out. All of this goes on a 300ci six Ford that is inverted.

The other thing is I couldnt access your website. All I got was "cant find server". Will keep trying though.

TKS
Mel

#### Jman

##### Site Developer
The other thing is I couldnt access your website. All I got was "can`t find server". Will keep trying though.
Try it now mel.

Jake

#### at7000ft

##### Member
Heard about Honda and Lycoming ?

I think the best thing that could happen for us lighter homebuilt builders would be a merging of engineering talent from the auto engine industy with the aircraft engine industry. I read an article about Honda working with Lycoming possibly on a new series of aircraft engines. It is hard to believe Honda is interested in spending their time on something like this, maybe they think the small general aviation market will really take off when the Sport Pilot thing finally happens.

And its also hard to believe that Lycoming would want to have new competiion for its own "stone knife and bear skin" technology line of engines. Of course they may be seeing how many engines Rotax is selling and have decided that they want a piece of the under 110 HP uncertified engine action. Actually I am surprised they haven't started an uncertified engine subsidiary disting off the old blueprints and making their old 65 horse engines again.

I have owned several Hondas and would love to see Honda create a line of small aircraft engines. Especially knowing that they have partnered with a knowledgable (although hopelessly 20th century) aircraft engine company on the project. I am currently looking at the Geo/Suzuki engine and the Raven Reduction drive. I know the Lycoming/Continentals are reliable but I can't justify spending year 2020 cash for year 1930 technology even though it would be the easiest path to take.

#### orion

##### Well-Known Member
I agree with you. My wife and I both drive Hondas and personally, I think the engines would be wonderful in an airplane application. As far as the company being interested in aviation, I wonder if it's just a reaction to the Toyota program. That was introduced several years ago and an aviation version of the engine did finally come out. I do not know the status currently but given the significant lack of information out there about it, I wonder if it didn't just fade away.

I know some time back the Toyota group actually developed a new airplane to act as a test bed and to be produced as a certified product however performance wise, it turned out to be a dog, delivering performance far below the advertised hype. (Actually knowing the design team, I'm not too surprised.)

One other thing I wanted to address regarding an earlier thread: several reduction drives out there are being based on the automotive gears found in automatic transmissions. These are planetary type and come all self contained in their own housing, with matching ring gear and sun gear. They are relatively easy to develop into a drive and have been tried by several suppliers of planetary systems.

However, I would caution how they are used. The environment these gears see in an autoamtic transmission is very benign - this is not necessarily the case in an aviation product. Given limits on torque and rpm, the sun and ring gears should be OK however the planets should cause a bit of concern.

The planetary gears are made from powdered metal, then sintered into shape. This is a process of taking the powder and subjecting it to high pressure and temperature, thus causing the powder to turn into a semi-liquid state, forming the part. The process is well known and although the quality controls are good, I wonder if they are dependable enough for our application.

I remember back in school we were shown a number of examples of part failure that were caused by incomplete formation of the powder to solid metal and/or part failure due to internal voids. I'm pretty confident the gears are OK for the automatic transmission application but for aircraft? I am not so sure.

About three years ago one company evaluated the gears for their engine configuration. The parts they were considering were either a four gear clusters or a six gear cluster from a Ford C-6 heavy duty transmission. In service the gears saw torques and rpms such as are put out by large trucks and other heavy duty applications. However, when analyzed by the engine company's transmission designers, they determined that the gears in an aviation application would be sutiable for engines producing no more than about 120 hp at 5,200 rpm. This was quite an eye opener not only for me but for several other similar projects.

If you're going to use this setup, my recommendation is to use the other components, but make your own planet gears from billet - throw the sintered gears away.

#### at7000ft

##### Member
Simplest reduction drive tech

The simplest solution that does the job is always the best. In the under 110 HP range that I am interested in two pulleys with multiple V-belts sure seems like the best way to go. I am sure this would be inadequate for 500 HP V-8s.

#### orion

##### Well-Known Member
"V" belts are a good way to develop a simple reduction drive but keep in mind several things: the "V" belt system is one of the lowest efficiency power transmission methods there is. While gears loose about one to two percent in efficiency per mesh, "V" belts can loose more than 15%. This loss is in the form of friction, which in turn is heat that has to be dissapaited. It also means that 15% less power is available for the propeller.

Also, as the system ages and the belts "break in", they can become loose, requiring periodic adjustments or a good idler/tensioner pulley. This deteriorating condition also causes further loss of efficiency.

Yes, "V" belts are simpler but keep in mind they do introduce significant drawbacks also. If this is the way you want to proceed, look at cog belts (such as teh HTD type) or other, more efficient systems.

#### at7000ft

##### Member
Good point, how efficient is a cog belt drive and would you want to use multiple belts for redundency like with v-belts?

#### orion

##### Well-Known Member
A well designed cog belt looses less than five percent per mesh. I have seen the manufacturer's data indicate that the losses can be as low as for gears (two to three percent).

The main thing to pay attention to in cog systems is the alignment of the two pulleys. Early attempts at cog reduction drives were of poor design and resulted in failure in a very short amount of time. I've seen several slapped together drives with lives of less than 15 hours. Well designed drives however can last over a thousand.

Multiple belts can of course add redundancy but will require the high level of alignment between all the cogs. Standard practice seems to be to just go with a single, wider belt instead.

#### mogren

##### Active Member
Better go back and check those belt vs gear #s. Planatary is the worst, gear to gear is next. . The best is poly v drive.
I like the quick change idea. i have a stack of gears and the noise is cool. ( Noise means loss of power.tho)
MM

#### orion

##### Well-Known Member
Hi;

Actually I don't have to check the numbers as I deal with them quite often on my own and in my business.

I don't know where you get your numbers but "V" belts consistently are the worst in power transfer. This doesn't require a whole lot of thought since if you consider that a "V" belt functions as a result of friction between the V's sides and the "notch" in the pulley, you will realize that in its work, the "V" belt continually loses energy and generates heat as a result of the surface contact.

Simple mechanical contact of gears or even cog belts is dramatically better through most functional realms.

Gear tooth-to-tooth contact results in only 1.5 to 3% loss per mesh. "V" belt loss can range from about 6% to more than 15%, depending on pulley configuration and type of contact. Yes, planetary gears can add up to a higher number but that is a function of how many planets you have in play. Many planets can reduce the load per contact, but the higher number of contacts does result in higher energy losses.

As a ballpark, some chains (silent type) and cog belts have about the same energy loss per drive. Chain must eliminate the heat through oiling while the belt requires a good air flow.

#### Johnny luvs Biplanes

##### Well-Known Member
What about the good old hot rodded GMC type blower belt (on dragsters). Would have though that they would use the easiest and most efficient system! Easy to design, manafacture, alter ratio's and can be fairly light. John

#### orion

##### Well-Known Member
The belt system used in the blower setup also depends on contact friction and has some limits on torque and rpm. Long term effects such as those encountered in aviation (or any other permanent setup) would have detrimental effects on belt life.

The reason they are used in the automotive industry is the same as you listed - they are simple, easy to install, easy to modify for the application, and relatively cheap.

#### MingoMontanaro

##### Member
Excellent Notes, Will take a note of it.

#### Robert Young

##### Member
belted redrive

Several helicopters are using V belts and poly-V belts in their primary drive reduction and in most cases the belt drive functions as the power clutch. The ones that come to mind are: Enstrom, Robinson, Hughes. If those belts were absorbing 15% of the power I would think they would melt. Those choppers are in the 160 to 225hp range and 15% is 30hp converted to heat or about 80,000 BTUs. My information shows Vbelts losing about 5%. What do the helicopter guys say?

#### Captain_John

##### Well-Known Member
Well, I personally wouldn't put a rubber belt on my own airscrew. I know that eggbeaters use them all the time, but my application is an airplane.

In that vein, I am entertaining an automobile recip engine. (John Slade extolls the virtues of the rotary design and that may sway my decision, but that is another story.) I am considering a Subaru 190hp 6 cylinder Eggenfellner engine or one similar of my own design. I see some stumbling blocks along the way that could pose a significant hurdle. One huge one is the redux drive.

I have not yet been convinced of a unit that offers sufficient reliability. For me I would demand billet gears lubricated with synthetic gear oil. I am open to other choices so long as I am confident in the design.

Something else that I was thinking about was the possibility of an overdrive unit, perhaps hydrostatically shifted? Now, this is a random thought of mine but if the gearing could smoothly change from takeoff mode (2:1) to a cruise mode (1:1) when less power was needed and fuel economy and engine life could be maximized the result would be a highly complicated, fragile and expensive proposition.

:roll:

But in all seriousness... why do we force auto engines to roar at high rpm when at cruise? Wouldn't it make more sense to operate them at the normal highway cruise rpm of 2,500 which coincidentally is the optimum airscrew operating speed?

How far away from reality are my thoughts?

CJ

#### Captain_John

##### Well-Known Member
Oh, and BTW...

I do understand that this overdrive idea would pay the price in final drive horsepower. I am sure the Soob which produces 190 hp at... oooooh 5,000 rpm will only be generating about 100 or so at 2,500 rpm.

This would be an acceptable trade off for some long distance flights where endurance is king over ground coverage.

CJ

##### Well-Known Member
You want confidence in you're redrive?
How about Tracy Crook's unit. He flys everything he makes, and has something like 1200 hrs on his own redrive. There are 30 or more of these units flying. See http://rotaryaviation.com for more details.

As for the gears idea - George Graham flys his rotary EZ with a stock transmission stuck in second gear. Works for him. Only one failure in hundreds of hours, and that was on the ground due to vibration from a rotor failure. I thought of copying his approach, but leaving all the gears in. It would be cool to have a clutch and gear stick, and change gears during the takeoff roll.

Sanity prevailed, and I bought Tracy's redrive.

#### Robert Young

##### Member
redrive

The fastest piston airplane to date, Rare Bear, uses the largest prop possible turning at a speed that will keep the prop tips below the critical mach number and still be able to absorb the power available. Rare Bear runs with a Wright 3350. I di not know what the gear reduction is on Rare Bear but as I recall thew B29 3350 had a gear reduction of about 3.3:1 and B29 props turned at less than 1000 rpm.

If you want prop efficiency you need diameter. There is little point in carrying around a lot of hp if it cannot be converted into thrust; therefore the redrive is necessary. The other benefit of low tip speeds is lower prop noise. I once had a Cessna 185 with a 300hp IO520 and an 88 inch seaplane prop. Great plane but at my homeport the airport manger asked if I would not take off at full power as he always got noise complaints. All I had to do was screw in a little more pitch to keep the revs down and I kept wide open throttle (WOT) so I was getting plenty of power. With the rpm reduced by about 200rpm I was giving up about 20hp to get the noise way down. I am not sure how much more thrust I would be getting with that additional 20hp because the tips were at or above the critical mach number. Now if I had a second gear in that airplane and a bigger propeller...

#### orion

##### Well-Known Member
I can certainly relate to your concern regarding redrive reliability. However, I think that with a bit of digging, the Eggenfellner may be OK. I actually don't know enough about the interior components of their drive but hearing from others that do, it seems to be one of good potential options for the Subaru package.

I agree with your requirement of billet-cut gears. The lubrication however does not have to be synthetic but it should be a pressure fed system (as opposed to a bath). Also, it would be a good idea to look at the finishing of the gear teeth. As a minimum, I would tend to specifiy an AGMA 12 finish, although I would prefer to see the aerospace standard of AGMA 16. Yes, it's a bit of overkill and would make the components a bit more expensive, but then the higher confidence level I would get from the higher precision of the gear geometry would make me feel better when in the air.

Also, keep in mind that with a redrive system, the longevity of the components will be highly a function of how you fly them. One of the key issues of redrive operation is to always remember that the teeth must maintain firm contact. As such, procedures such as descent under partial power must be minimized or if possible, avoided. This is one of the most damagiong aspects of flight to a gearbox.

When descending under partial power the teeth of the box may not necessarily be in full mesh, as the air load on the prop may actually tend to at times pull it ahead of the engine. When the contact force of the gear teeth is reduced, there is a strong chance of chatter. At that point there could be two types of damage, both potentially catastrophic.

Both are a function of the force of impact as the teeth switch between contact on the front and back faces. The impact under certain conditions could create material stresses higher than what the teeth see even under full throttle. This could crack teeth at the base, or chip the faces or edges.

The second aspect is the possible damage caused to the face of the tooth. These are a function of the locallized bearing stresses which cause small levels of damage to the tooth face itself. This, while it is not immediately catastrophic, could introduce stress concetrations into the tooth structure, which could eventually lead to tooth failure.

Material failure as a result of high contact (bearing) loads however is detectable since the running noise level of the gear box will increase as the damage gets worse.

For more info on reduction drive issues I would suggest talking to someone who maintains geared Lycoming or Continental engines. I'm sure they'd have a few stories for you, as well as many hints for good operating practices.

A couple of other points regarding the above posts. The reason we tend to run the automotive conversions at such high rpm, for those who don't know, is just to get the necessary horsepower out of them. Horsepower, in the simplest of terms, is a function of torque and rpm. That in turn is a function of displacement and the amount of fuel we can stuff in. The automotive engines tend to have realtively small displacements so to get the horsepower, we need to turn them higher. Keep in mind that an O-360 produces its 180 or so horsepower at 2,700 rpm. It could produce a lot more if we turned it faster but then you'd have a less efficient engine since not all the fuel/air mixture could fully burn given the large area of the engine's piston face.

An automotive V-8 however is of about the same displacement but we tend to try to get a lot more horsepower out of it per cubic inch than what we demand of the Lycoming. Since it has pretty much the same displacement as the O-360 (let's assume were talking about a GM 350), the only way we can get more power out of it is by stuffing in more fuel thus increasing the torque (turbocharging), or by turning it faster.

Yes, we could design a sytem that is direct drive but for the given lower rpm, that would require quite a bit of boost to get the higher torque and be able to deliver a reasonable power to weight fraction.

We could also do the direct drive so that it turns a higher rpm but then we would need to design a prop that is specifically lofted for the higher speed. It is possible and contrary to conventional thinking, efficiency is not necessarily a pure function of diameter. However keep in mind that to do this one would have the additional cost of the engineering and development needed to develop the new prop. It is for this reason that we try to develop engines that operate around the more conventionsl props, most of which have design baselines that go back more than fifty years and are used by myriads of engines (and so are more affordable).

And a last point - don't bother with the idea of "shifting" gears. When you examine the needs of the prop and the flight envelope, it does not work.

Furthermore, such a system just introduces more complexity and weight to the system.

This is why you have controllable pitch or constant-speed props.

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